I. Field of the Invention
The present invention relates to communications systems. More particularly, the present invention relates to a novel and improved method for independent closed loop power control of subchannels in a spread spectrum communication system.
II. Description of the Related Art
In a code division multiple access (CDMA) spread spectrum communication system, a common frequency band is used for communication with all base stations within that system. An example of such a system is described in the TIA/EIA Interim Standard IS-95-A entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, incorporated herein by reference. The generation and receipt of CDMA signals is disclosed in U.S. Pat. No. 4,401,307 entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS USING SATELLITE OR TERRESTRIAL REPEATERS” and in U.S. Pat. No. 5,103,459 entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM” both of which are assigned to the assignee of the present invention and incorporated herein by reference.
Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a high rate pseudonoise (PN) code. A PN code is used to modulate signals transmitted from the base stations and the remote stations. Signals from different base stations can be separately received at the receiving station by discrimination of the unique time offset that is introduced in the PN codes assigned to each base station. The high rate PN modulation also allows the receiving station to receive a signal from a single transmission station where the signal has traveled over distinct propagation paths. Demodulation of multiple signals is disclosed in U.S. Pat. No. 5,490,165 entitled “DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS” and in U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, both of which are assigned to the assignee of the present invention and incorporated herein by reference.
The IS-95 Over-the-Air (OTA) Interface Standard defines a set of RF signal modulation procedures for implementing a digital cellular telephone system. The IS-95 standard, and its derivatives, such as IS-95A and ANSI J-STD-008 (referred to collectively as the IS-95 standard), are promulgated by the Telecommunications Industry Association (TIA) to insure the operability between telecommunications equipment manufactured by different vendors.
The IS-95 standard has received enthusiastic reception because it uses the available RF bandwidth more efficiently than previously existing cellular telephone technologies. This increased efficiency is provided by using CDMA signal processing techniques in combination with extensive transmit power control to increase the frequency reuse of a cellular telephone system.
FIG. 1 illustrates a digital cellular telephone system configured in a manner consistent with the use of IS-95. During operation, telephone calls and other communications are conducted by exchanging data between remote stations 1 (generally cellular telephones) and base stations 2 using RF signals. Communications are further conducted from base stations 2 through base station controllers (BSC) 4 and mobile switching center (MSC) 6 to either public switch telephone network (PSTN) 8, or to another base station for transmission to another remote station 1. BSCs 4 and MSC 6 typically provide mobility control, call processing, and call routing functionality.
The RF signal transmitted from a base station 2 to a set of remote stations 1 is referred to as the forward link, and the RF signal transmitted from remote stations 1 to a base station 2 is referred to as the reverse link. The IS-95 standard calls for remote stations 1 to provide telecommunications service by transmitting user data such as digitized voice data via the reverse link signal. The reverse link signal is comprised of a single traffic channel, and therefore is often referred to as a “non-coherent” signal because it does not include a pilot channel, and as such cannot be coherently demodulated.
Within the reverse link signal, user data is transmitted at a maximum data rate of 8.6 or 13.35 kbps, depending on which rate set from a set of rate sets provided by IS-95 is selected. The use of a single channel, non-coherent, reverse link signal simplifies the implementation of an IS-95 cellular telephone system by eliminating the need for synchronization between a set of remote stations 1 communicating with a single base station 2.
As mentioned above, IS-95 incorporates extensive transmit power control in order to more efficiently utilize the available RF bandwidth. In accordance with IS-95, this power control is performed by measuring the received signal strength and quality of the reverse link traffic channel when received at the base station and generating a power control command based on that measurement. The power control command is transmitted to the remote station via the forward link signal. The remote station responds to the power control command by increasing or decreasing the transmit power of the reverse link signal based on the power control command. This power control method is referred to as closed loop power control. The design of closed loop power control in a CDMA communication system is described in U.S. Pat. No. 5,056,109, entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM”, which is assigned to the assignee of the present invention and incorporated by reference herein.
In IS-95 systems, the power control adjustment is performed repeatedly at rates on the order of 800 times per second in order to maintain the reverse link signal transmit power at the minimum necessary to conduct communications. Additionally, IS-95 also calls for transmit duty cycle of the reverse link signal to be adjusted in response to changes in voice activity by varying the transmit duty cycle in 20 millisecond increments. Thus, when the transmit duty cycle is lowered, the remote station transmits at either the set point, or the transmission is gated and the remote station does not transmit at all. During periods when the transmission is gated, the base station generates false power control increase commands because the reverse link signal is not detected. Since the remote station knows when its transmissions were gated, it can ignore corresponding increase commands since they are known to be false.
To satisfy the ever increasing demand to transmit digital data created by networking technologies, such as the worldwide web, a more complex higher rate multi-channel coherent reverse link signal is provided in co-pending U.S. Pat. No. 5,930,230 (the '230 patent) entitled “High Data Rate CDMA Wireless Communications System” issued Jul. 27, 1999, assigned to the assignee of the present invention and incorporated herein by reference. The above referenced patent describes a system wherein a set of individually gain adjusted channels are formed via the use of a set of orthogonal subchannel codes. Data to be transmitted via one of the transmit channels is modulated with one of the subchannel codes, gain adjusted, and summed with data modulated using the other subchannel codes. The resulting summed data is modulated using a user long code and a pseudorandom spreading code (PN code) and upconverted for transmission. In particular, the above referenced patent describes a reverse link signal made up of Walsh sequence modulated subchannels including at least one traffic subchannel, a power control subchannel, and a pilot subchannel
A multi-channel reverse link increases flexibility by allowing different types of data to be transmitted simultaneously. Providing a pilot subchannel facilitates coherent processing of the reverse link signal at the base station which improves the performance of the link. To facilitate power control, time tracking and frequency tracking, it may be desirable to keep the average received pilot signal power to noise ratio (SNR) at a constant level. Note that in a CDMA based system, effective power control is essential to achieving a high system capacity. Usually, power control is divided into two parts, an open loop and a closed loop. In open loop power control, the mobile station measures the received forward link signal for a predetermined time period and adjusts its transmit power in response to changes in the received forward link power. The open loop power control as implemented in IS-95 systems is fairly slow and takes care of the long term channel variations (known as the corner effect). The closed loop power control as described previously is faster and tries to compensate for the effects of fading.
In IS-95 based CDMA systems, the closed loop power control is also used to drive the reverse link to a desired setpoint. For example, a frame error rate (FER) of 1% may be desired. If the FER is too low, an increase in reverse link power is necessary to reduce the error rate. On the other hand, if the FER is lower than the desired setpoint, the reverse link power can be reduced. Reducing the reverse link power reduces interference generated and thus has a direct positive effect on the other users in the system. Maximum capacity is reached in a CDMA system when every user is transmitting at the setpoint and therefore the minimum power required to achieve the desired error rate.
The operating setpoint of the system can be modified by changing the power control decision threshold at the base station. As a consequence, the total average received power of the reverse link will converge to a new value. This power control mechanism affects the total transmitted power. However, if this technique is applied to a system employing a plurality of subchannels as provided for in the '230 patent, the relative strengths of each subchannel are not changed as the total transmit power is modified. For example, upon reaching a satisfactory power level in terms of received pilot subchannel power, any subsequent variation of transmit power to modify the received FER for a data subchannel will affect the pilot power, and vice versa. Since it is likely that different types of data which occupy separate subchannels will have different requirements, it is desirable to be able to control the transmitted power of each subchannel independently.